Optical module and manufacturing method of optical module

ABSTRACT

An optical module according to the present invention has a stem having an interconnection terminal; an optical device positioned on the stem and electrically connected to the interconnection terminal; a cap fixed onto the stem so as to enclose the optical device within the cap; and a lens for optically coupling the optical device with an optical member positioned outside the cap. As a feature of this optical module, the cap is manufactured with a high degree of precision so that the height of the cap with respect to the stem can be used as a positional reference in the direction of the optical axis of the optical system, which consists of the optical device, the lens, and the optical member extending outside the cap. In addition, the shape of the side wall of the cap is designed so that it can function as a reference for maintaining the coaxiality between the photo detector and the lens. The stem is fixed to the cap at the proper position a high precision.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] This invention relates to an optical module used to transmit andreceive optical signals in optical communication systems.

[0003] 2. Description of Related Art

[0004] In general, optical signals which have been transmitted throughoptical fibers are received by the input ports of receivers, andconverted into electric signals by photo detector modules using, forexample, photo diodes (PD).

[0005] The size of the detection area of the photo detector used in sucha photo detector module is very small having a diameter of about 80 μm.In order to efficiently receive the optical beam emitted from theoptical fiber into the photo detector, a collecting lens is typicallyused. For example, a technique of attaching a ball lens directly to thecap of the photo detector is known as a low-cost light-collectingmethod.

[0006]FIG. 5 illustrates an example of this type of photo detectormodule. This photo detector module comprises a commercially availablestem 2, which is generally referred to as a TO-46 type stem, and a photodetector 1 mounted on the stem 2. The stem 2 is fixed to a cap 3 so thatthe photo detector 1 is accommodated in the cap 3. A ball lens 4 made oflow-temperature-melting glass is attached to the middle of the cap. Thecap 3 is fixed to a holder 5 by a bond 5, such as adhesive or solder.

[0007] An optical fiber 7 is connected to the holder 5 via a sleeve 9.To be more precise, the optical fiber 7 is inserted in a ferrule 8, andthe ferrule 8 is inserted in and fixed to the sleeve 9 by YAG welding.As a result of the YAG welding, nuggets 10 are formed at the boundariesbetween the holder 5 and the ferrule 8, and at the boundaries betweenthe ferrule 8 and the sleeve 9.

[0008] In assembling the conventional photo detector, first, the photodetector 1 is bonded to the stem 2. Then, the stem 2 is fitted into thecap 3, to which the ball lens 4 has already been fixed, and the stem 2is fixed to the cap 3 by resistance-welding.

[0009] Then, the holder 5 is fixed to the cap 3 at the end opposite thestem 2 by a bond 6. The cap 3 can not be directly YAG welded because thecap 3 is press-processed.

[0010] Finally, the optical fiber 7 is positioned in the optimalposition by an optical alignment technique, and then fixed to the holder5, via the sleeve 9, by YAG welding.

[0011] When this photo detector module is used for high-speedtransmission in a STM system, the optical signal which has beenpropagated through the optical fiber 7 must be prevented from beingreflected by the surface of the photo detector 1 back to thetransmission path. For this reason, the end surfaces of the ferrule 8and the optical fiber 7, which surface the ball lens 4, are slanted bygrinding or polishing for the purpose of inclining the incident angle ofthe beam onto the photo detector 1, and of reducing the return light tothe optical fiber 7.

[0012] In particular, if the inclination of the end surface of theoptical fiber 7 is set to about 12 degrees, the incident angle of thebeam on the photo detector 1 becomes about 6 degrees, which can reducethe reflected component of the signal light to less than −40 dB.

[0013] However, this arrangement causes a large variation in thequantity of the reflected light because the coaxiality between the photodetector 1 and the ball lens 4 is lost.

[0014] FIGS. 6(1) through 6(3) show the optical coupling and thecoaxiality between the photo detector 1 and the ball lens 4.

[0015]FIG. 6(1) illustrates the optical connection at the optimalposition with little separation at the ball lens 4. In this arrangement,the light emitted from the optical fiber 7 strikes the photo detector 1at an optimal incident angle, and the amount of the reflected light backto the optical fiber 7 via the ball lens 4 is greatly reduced. In thisarrangement, the orientation of the end surface of the optical fiber 7does not affect the physical properties of the optical connectionbetween the ball lens 4 and the photo detector 1.

[0016] However, if the optical axes of the photo detector 1 and the balllens 4 are offset from each other, as shown in FIGS. 6(2) and 6(3), thequantity of reflected light increases, while the coupling efficiencydecreases.

[0017] For example, if the separation between the photo detector 1 andthe ball lens 4 and the orientation of the slanted end surface of theoptical fiber 7 satisfy particular conditions (that is, if theorientation of the end surface of the optical fiber 7 is 12 degrees, andif the separation is about 0.11 mm, as shown in FIG. 6(2)), then aproblem arises. In the example of FIG. 6(2), if the optical fiber 7 isaligned to the optimal position, the light which exits obliquely fromthe optical fiber 7 strikes the surface of the photo detector 1 at anormal angle. The normal incident light is reflected by the photodetector, and returns along the same path as the incident light to theoptical fiber 7. The amount of reflected light may reach a maximum inthis arrangement.

[0018] On the other hand, as shown in FIG. 6(3), if the orientation ofthe inclined end surface of the optical fiber 6 is opposite theorientation shown in FIG. 6(2), the quantity of reflected light isreduced. However, in this arrangement, the light emitted from theoptical fiber 7 passes through the ball lens 4 at a position offset fromthe center axis of the ball lens 4. As a result, the sphericalaberration of the ball lens 4 adversely affects the system, whichreduces the optical coupling efficiency and causes the light-collectingability of the photo detector to deteriorate.

[0019] In addition to these problems, the coaxiability between the photodetector 1 and the ball lens 4 of the conventional photo detector modulemay be offset by 0.25 mm at most due to the following factors:

[0020] (1) Because there is no target or alignment mark on the stem 2,the photo detector 1 can not be precisely bonded;

[0021] (2) Because the cap 3 is press-processed, its dimensionalstability is insufficient and, accordingly, a large clearance isrequired at the portion for receiving the stem 2, which causes theoptical axes of the stem 2 and the cap 3 to be offset from each otherduring the resistance welding step; and

[0022] (3) It is mechanically difficult to accurately fix the ball lens4 in the center of the cap 3.

[0023] If the separation between the photo detector 1 and the ball lens4 is large, the amount of reflected light and the coupling efficiencyvary greatly with the slanting orientation of the end surface of theoptical fiber 7. In order to achieve a predetermined desiredperformance, employing an optical alignment technique for setting theorientation of the slanting end surface of the optical fiber 7 at theoptimal angle, while monitoring the reflected light and the couplingefficiency, is indispensable. However, such an optical alignmenttechnique requires a number of steps, and the production yield islowered as a result.

[0024] In addition, it is also difficult in the conventional photodetector module to precisely position the holder 5 with respect to thecap 3 along the optical axis (i.e., the z-axis) because of the existenceof adhesive or solder. Since the optical fiber 7 must be preciselypositioned both in the direction perpendicular to the optical axis andin the direcxtion parallel to the optical axis, the ferrule 8 and theholder 5 need to be secured at two positions via the sleeve 9 by YAGwelding.

[0025] Furthermore, an adhesive or solder is also used to fix the holder5 to the cap 3, which is not preferable for maintaining the reliabilityof the optical module for a long period of time.

SUMMARY OF THE INVENTION

[0026] Therefore, it is an object of the invention to overcome theseproblems in the prior art, and to provide an optical module whichcomprises a stem having an interconnection terminal; an optical devicepositioned on the stem and electrically connected to the interconnectionterminal; a cap fixed onto the stem so as to enclose the optical devicewithin the cap; and a lens for optically coupling the optical devicewith an optical member positioned outside the cap. As a feature of thisoptical module, the cap is manufactured with a high degree of precisionso that the height of the cap with respect to the stem can be used as apositional reference in the direction of the optical axis of the opticalsystem, which consists of the optical device, the lens, and the opticalmember extending outside the cap.

[0027] In addition, the shape of the side wall of the cap is designed sothat it can function as a reference for maintaining the coaxialitybetween the photo detector and the lens. The stem is fixed to the cap atthe proper position a high precision.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects and features of the invention willbecome more apparent from the following detailed description of thepreferred embodiments with reference to the attached drawings, wherein:

[0029]FIG. 1 is a cross-sectional view of the optical module accordingto an embodiment of the invention;

[0030]FIG. 2 is an exploded perspective view of the optical module,which is used to explain the process of assembling the optical module ofthe invention;

[0031]FIG. 3 illustrates how the stem is fixed to a base;

[0032]FIG. 4 illustrates how the stem is fixed to the cap;

[0033]FIG. 5 illustrates a conventional photo detector module; and

[0034]FIG. 6 illustrates the relationship between the optical couplingstate and the coaxiability between the photo detector and the ball lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The preferred embodiments of the invention will now be describedin detail with reference to the attached drawings. FIG. 1 is across-sectional view of the optical module according to a preferredembodiment of the invention, and FIG. 2 is an exploded perspective viewof the optical module of this embodiment, which shows how this opticalmodule is assembled.

[0036] In this embodiment, a photo detector 11 consisting of a photodiode (PD) is used as the optical device. The photo detector 11 ismounted on a pad provided to the surface of a base 13 made of ceramic.The base 13 is fixed to a metallic stem 12 which is commerciallyavailable under the trade name of “TO-46 type”. The base 13 is capableof receiving a simple circuit including, for example, a pre-amplifier 14and a capacitor 15, in addition to the photo detector 11. In thisembodiment, these circuitry elements are electrically connected to theinterconnection terminal of the stem 12, whereby a photo detector modulehaving a built-in pre-amplifier is formed.

[0037] A metallic cap 16 is fixed to the stem 12 by resistance welding,so that the photo detector 11 on the stem 12 is enclosed within the cap16. A ball lens 17 is fixed in the middle of the cap 16 bylow-temperature-melting glass. The ball lens 17 is used to opticallycouple the photo detector 11 with an optical member positioned outsidethe cap 16.

[0038] The cap 16 is formed with a high degree of precision by cuttingprocesses, unlike the conventional cap which is formed by pressprocesses. The height of the cap 16 with respect to the stem 12 is usedas a positional reference in the z-direction along the optical axis ofthe system extending from the ball lens 17 to the external opticalmember. In particular, the top wall of the cap 16 is shaped so that thetop surface of the cap 16 is positioned a predetermined distance fromthe ball lens 17. In other words, as shown in FIG. 1, the thickness ofthe top wall of the cap 16 is adjusted so that the focal point of theball lens 17 is positioned below the top surface of the cap 16.

[0039] The external optical member is an optical fiber 18 in thisembodiment. The end surface of the optical fiber 18 is optically coupledto the cap 16. The optical fiber 18 is held by a ferrule 19 whichfunctions as a fiber holder. The end surface of the ferrule 19 is fixedto the top surface of the cap 16 by YAG welding, whereby the opticalfiber 18 is also secured to the cap 16. The end surface of the opticalfiber 18 projects from the end surface of the ferrule 19 by apredetermined length along the optical axis, such that the focal pointof the ball lens 16 is located within the plane of the end surface ofthe optical fiber 18. As a result of the YAG welding of the ferrule 19to the cap 16, a nugget 20 is formed at the boundary between the ferrule19 and the cap 16.

[0040] Next, the process of assembling this photo detector module willbe described. Many techniques for achieving highly precise alignmentbetween the photo detector 11 and the ball lens 17 are employed in thisinvention.

[0041] First, the base 13, on which a pad for receiving the photodetector 11 is formed, is mounted on the stem 12. Then, the photodetector 11 is bonded onto the pad. The stem 12 is fitted into the cap16, to which the ball lens 17 has already been secured, and then thestem 12 is fixed to the cap 16 by resistance welding. Finally, after theoptical fiber 18 held by the ferrule 19 is optically aligned with thephoto detector 11, the ferrule 19 is secured to the cap 16 by YAGwelding.

[0042] The factors affecting the coaxiability between the photo detector11 and the ball lens 17 are listed below.

[0043] (1) Precision of the patterning of the pad on the base 13

[0044] (2) Positional accuracy of mounting the base 13 on the stem 12

[0045] (3) Accuracy of bonding the photo detector 11 onto the base 13

[0046] (4) Accuracy in resistance-welding the cap 16 to the stem 12

[0047] (5) Coaxiality between the cap 16 and the ball lens 17

[0048] The separations due to these factors are cumulated, which resultsin a total separation between the photo detector 11 and the ball lens17.

[0049] Each of the above-listed factors will now be explained in moredetail.

[0050] (1) Precision of the patterning pad on the base 13

[0051] The base 13 is made of ceramic, and a pad for receiving the photodetector 11 is formed on the base 13 by etching using a photomask. Theprecision of the patterning of the pad is determined by the precision ofthe alignment of the photomask.

[0052] (2) Positional accuracy of mounting the base 13 on the stem 12

[0053] In order to mount the base 13 on the stem 12 at the properposition with a high degree of precision, the base 13 and the stem 12are coupled using a coupling device 21 shown in FIG. 3, whichillustrates the coupling steps. The upper half of FIG. 3 showscross-sectional views, while the lower half shows the correspondingperspective views.

[0054] The coupling device 21 has a guide hole 21-1 whose diameter isslightly larger than the outer diameter of the base 13, and a guide hole21-2 whose diameter is slightly larger than the diameter of thecomponent-mounting area of the stem 12. The guide holes 21-1 and 21-2are concentric.

[0055] In coupling the base 13 to the stem 12, the base 13 is set intothe guide hole 21-1 of the coupling device 21, as shown in FIG. 3(1),and a bond (not shown), such as solder, is put on the top surface of thebase 13.

[0056] Then, the stem 12 is fitted into the guide hole 21-2 so that thecomponent-mounting area of the stem 12 comes into contact with the topsurface of the base 13 with the bond therebetween, as shown in FIG.3(2).

[0057] The coupling device 21, in which the base 13, the bond, and thestem 12 are set, is heated up, as shown in FIG. 3(3), whereby the base13 and the stem 12 are bonded to each other.

[0058] Finally, the base 13 and the stem 12, which are now bonded intoone unit, are removed from the coupling device 21, as shown in FIG.3(4).

[0059] In this process, the coaxiality between the base 13 and the stem12 depends on the gap between the guide hole 21-1 and the base 13, andthe gap between the guide hole 21-2 and the stem 12.

[0060] In this embodiment, a commercially available TO-46-type stem isused. This stem 12 is press-processed, and the outer diameter of thecomponent-mounting part, including the systematic error (or tolerance),is φ4.2±0.025 mm. The base 13 is made of ceramic, and its outer diametertolerance depends on the amount of contraction during the bakingprocess. The actually measured tolerance of the base 13 is ±0.03 mm. Thediameters of the guide holes 21-1 and 21-2 of the coupling device 21 areset slightly larger than the maximum tolerances of the base 13 and thestem 12, respectively. Accordingly, the maximum separation caused in theworst case is about 0.065 mm. Since the circumference of the base 13must not stick out beyond the circumference of the stem 12 even in theworst case, the outer diameter of the base 13 is set slightly smallerthan the outer diameter of the stem 12.

[0061] (3) Accuracy of bonding of the photo detector 11 onto the base 13

[0062] The photo detector 11 is mounted on the pad formed on the base 13by an ordinary bonding process. The uncertainty of the positioning ofthe photo detector 11 with respect to the pad is about 0.05 mm.

[0063] (4)Accuracy in resistance-welding the cap 16 to the stem 12

[0064]FIG. 4 illustrates how the cap 16 is fixed to the stem 12. Thestem 12 is fitted into the cap 16 so that the side wall of thecomponent-mounting part of the stem 12 comes into contact with the innersurface of the edge of the cap 16. The coaxiality between the cap 16 andthe stem 12 depends on the gap between the outer diameter of thecomponent-mounting part of the stem 12 and the inner diameter of the cap16.

[0065] As has been explained above, the outer diameter of thecomponent-mounting part of the stem 12 is φ4.2±0.025 mm. The cap 16 ismanufactured by cutting processes, and the uncertainty in its innerdiameter is ±0.025 mm or less. Accordingly, if the inner diameter of thecap 16 is set to φ4.25±0.025 mm, the maximum separation between the cap16 and the stem 12 is only 0.050 mm in the worst case.

[0066] The flange 23 a of the stem 12 and the flange 23 b of the cap 16are fixed to each other by resistance welding. The inner corner of theflange 23 b of the cap 16 is chamfered, as indicated by the numericalreference 22 in FIG. 4. In general, the boundary between the flange 23 aand the side wall of the press-processed stem 12 becomes distort duringthe resistance welding. However, the chamfer 22 can prevent suchdistortion from adversely affecting the bonding of the stem 12 and thecap 16. The shape of the chamfer 22 is not limited to the example shownin FIG. 4, and any shapes can be selected as long as the flange 23 b ofthe cap 16 does not come into contact directly with the distortion ofthe stem 12.

[0067] In the example shown in FIG. 4, the diameter of the flange 23 bof the cap 16 is set larger than the diameter of the flange 23 a of thestem 12. The flange 23 b of the cap 16 is also thicker than the flange23 a of the stem 12 because the cap 16 is formed by cutting processes.During the resistance welding, the flange 23 b of the cap 16 and theflange 23 a of the stem 12 are pressed against each other at anappropriate pressure, and the flange 23 b of the cap 16, which isthicker and has a greater diameter than the flange 23 a of the stem 12,can reliably receive and hold the stem 12 during the resistance welding.

[0068] (5) Coaxiality between the cap 16 and the ball lens 17

[0069] The ball lens 17 is secured to the cap 16 bylow-temperature-melding glass using an ordinary alignment device. Aconventional alignment device is sufficient to reduce errors in thecoaxiality and the position along the optical axis.

[0070] As has been described, each component is positioned and alignedwith a high degree of precision, and the overall coaxiality between thephoto detector 11 and the ball lens 17 is greatly improved as comparedwith the conventional art.

[0071] In this embodiment, the coaxiality between the photo detector 11and the ball lens 17 is within 0.1 mm. It was confirmed by experimentthat, with this arrangement, the amount of reflected light and thecoupling efficiency do not greatly change with variation of the slantingangles and the orientations of the end surfaces of the optical fiber 18and the ferrule 19. The troublesome steps performed in the prior art ofadjusting the orientation of the ferrule 19 to the optimal angle, whilemonitoring the physical properties of the optical system, can beeliminated, and the work efficiency can thereby be improved.

[0072] The accuracy of positioning along the optical axis is alsoimproved because the cap 16 is manufactured by cutting processes, andthe height of the cap 16 can be set to the most appropriate value so asto satisfy the predetermined optical-coupling conditions. In otherwords, the cap 16 processed by cutting and having a desired height inaccordance with the predetermined conditions is bonded directly to thestem 12 by resistance welding, without requiring further positioning oradjustment along the optical axis. This is a great advantage over theconventional press-processed cap.

[0073] Thus, the cap 16 functions as a positional reference along theoptical axis of the optical system, which consists of the optical deviceformed on the stem, the ball lens, and the external optical member(i.e., the optical fiber). This facilitates the optical alignment of theoptical fiber, and the reflected light from the top surface of the photodetector can stably be reduced.

[0074] In addition, adhesive or solder used in the prior art iseliminated, and the reliability and mechanical accuracy of the entiremodule can be improved.

[0075] Although a photo detector module and its manufacturing methodhave been described, the present invention can also be applied to alight-emitting module. In this case, a ball lens is incorporated into acap processed by cutting, and the cap is resistance-welded to a stemusing the cap's height as a positioning reference along the opticalaxis. Accordingly, a light-emitting module using a laser diode (LD) isalso included in the scope of the invention.

What is claimed is:
 1. An optical module comprising: a stem having aninterconnection terminal; an optical device positioned on the stem andelectrically connected to the interconnection terminal; a cap fixed ontothe stem so as to enclose the optical device within the cap; and a lensfor optically coupling the optical device with an optical memberpositioned outside the cap, wherein, a height of the cap with respect tothe stem can be used as a positional reference in a direction of anoptical axis of an optical system.
 2. An optical module according toclaim 1 , wherein the stem is fixed to the cap and shape of a sidewallof the cap is designed so that it can function as a reference formaintaining a coaxiality between the optical device and the lens.
 3. Anoptical module according to claim 1 , wherein a top surface of the capis positioned a predetermined distance from the lens.
 4. An opticalmodule according to claim 3 , wherein a focal point of the lens ispositioned below the top surface of the cap.
 5. An optical moduleaccording to claim 3 , the optical member comprising: an optical fiberhaving an end surface which is optically coupled to the optical device;a fiber holder for holding the optical fiber and has an end surfacewhich is fixed to the top surface of the cap; and the end surface of theoptical fiber is aligned from the end surface of the fiber holder by apredetermined length along the optical axis.
 6. An optical moduleaccording to claim 3 , wherein the stem is a metallic stem, the cap is ametallic cap, and the metallic cap is fixed to the stem by resistancewelding.
 7. An optical module according to claim 1 , wherein the opticaldevice is a photo detector.
 8. An optical module according to claim 1 ,wherein the optical system consists of the optical device, the lens, andthe optical member.
 9. A method of manufacturing an optical modulecomprising the steps of: positioning an optical device on a stem whichhas an interconnection terminal and electrically connecting the opticaldevice to the interconnection terminal; positioning an lens in apredetermined position of a cap which has a predetermined height; andfixing the cap onto the stem so as to enclose the optical device withinthe cap, so that the height of the cap with respect to the stem can beused as a positional reference in a direction of an optical axis of anoptical system.
 10. A method of manufacturing an optical moduleaccording to claim 9 , wherein the stem is fixed to the cap and shape ofa side wall of the cap is designed so that it can function as areference for maintaining a coaxiality between the optical device andthe lens.
 11. A method of manufacturing an optical module according toclaim 9 , further comprising the steps of: holding an optical fiberwhich has an end surface by a holder, so that the end surface of theoptical fiber is located a predetermined position from an end surface ofthe holder; and fixing the end surface of the holder to the top surfaceof the cap.
 12. A method of manufacturing an optical module according toclaim 11 , wherein a focal point of the lens is positioned below the topsurface of the cap.
 13. A method of manufacturing an optical moduleaccording to claim 9 , wherein the optical device is set on a base andthe base is bonded to the stem, and the step of positioning the opticaldevice on the stem comprises fitting the base and the stem into apredetermined guide hole of a coupling device and heating up thecoupling device for coupling the base and the stem.
 14. A method ofmanufacturing an optical module according to claim 9 , wherein an innercorner of the cap is chamfered.
 15. A method of manufacturing an opticalmodule according to claim 11 , wherein the stem is a metallic stem, thecap is a metallic cap, and the metallic cap is fixed to the stem byresistance welding.
 16. A method of manufacturing an optical moduleaccording to claim 15 , wherein the stem has a first flange for theresistance welding, the cap has a second flange for the resistancewelding, and a diameter of the second flange is set larger than adiameter of the first flange.